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<br />38 <br /> <br />BIG SANDY RIVER SALINITY CONTROL UNIT, WYOMING <br /> <br />l\) <br />~ The Big Sandy River located in southwestern wyoming begins in the <br />~ Wind River Mountains where water quality is very good. Water <br />~ inflow to Big Sandy River Reservoir for irrigating about 16,000 <br />acres in Eden Valley has an average salinity concentration of <br />109 mg/L. seepage return below the irrigation project averages <br />4000 to 5500 mg/L. Average salinity concentration of surface <br />flow and seepage in Big Sandy outflow to the Green River is 2200 <br />mg\L (USDA 1980 & 1987), <br /> <br />USBR drilled 92 wells between 1975 and 1978 to locate and <br />characterize the various saline aquifers. Drilling investigations <br />have shown that the shallow aquifer under the irrigated area is <br />the source of saline return flows. The formations in the Green <br />River Basin is a succession of fluvial and lacustrine sediments. <br />Lake Gosiute formed with fluvial deposits of the Bridger <br />Formation covering the Green River Formation. Excessive <br />irrigation results in seepage and deep percolation into the <br />Bridger and Green River Formation. This allows ground water <br />recharge to interact with the salts and saline facies and results <br />in high salt loading, The bulk of the aquifer discharge occurs <br />along a 15 mile reach of the Big Sandy River in the form of <br />drains, seeps, springs, and flowing wells The total dissolved <br />solids concentration varies from 4000 mg/L to 5500 mg/L in the 25 <br />cfs of subsurface return flow. Salt loading of this irrigation <br />return flow is , to 7 tons per acre-foot. <br /> <br />Based on the historic water supply and the irrigation of 15,700 <br />acres with a present onfarm efficiency of 39% and a project <br />efficiency of 32% the water and salt budgets show 157,570 tons of <br />salt in the Big Sandy River, See table 4-5 and Figure 4-1a. <br />Under present conditions, it is estimated that 142,250 tons comes <br />from irrigation and 15,320 tons comes from runoff, erosion, and <br />natural seeps, The salt loading factor computes to g,O tons per <br />irrigated acre, <br /> <br />Implementation of the selected plan (primarily low pressure <br />sprinkler irrigation systems) will reduce salt loading to the Big <br />Sandy River from irrigation by 53,000 tons annually. This will <br />reduce the salinity concentration in the Green River by 27 mg/L <br />and decrease the salinity concentration by 5 mg/L at Imperial Dam <br />on the lower Colorado River, <br />